The invention relates to a process and apparatus for converting solar energy into heat by means of a solar collector consisting of a plurality of mutually coupled collector elements placed on the outside of a building or the like and connected to a heat transfer fluid network. More particularly each collector element includes a collector channel located at the bottom of a collector chamber delimited by a transparent cover, the collector channel being normally connected via a connecting line consisting of a forward branch and return branch to a high-temperature reservoir until a control valve arrangement is reversed and the collector channel is disconnected from the high-temperature reservoir and connected to a low-temperature reservoir for heat pump operation when the temperature of the circulating heat transfer fluid falls below a minimum value predetermined by the temperature in the high-temperature reservoir and measured by means of a temperature sensor associated with the collector channel.
An apparatus of this type is of relevance above all for electrical multivalent domestic-heat-control, with solar energy preferably being used, in which the solar collector is associated with an absorber facing away from the sun, thus forming a collector referred to as "hybrid collector" or "all-weather collector", respectively.
In a known process of this type, which makes use of the apparatus disclosed in European Patent Application 0,054,729, the heat transfer fluid to be found, when the collector channel is connected to the low-temperature reservoir, in the disconnected idle part of the line running to the high-temperature reservoir can cool down to the ambient temperature. As a consequence, with increasing sunshine and transition to high-temperature reservoir operation, first the comparatively cold heat transfer fluid flows into the high-temperature reservoir before the heated follow-on heat-transfer fluid exiting the collector channel which is exposed to the sunlight enters the high-temperature reservoir. On the other hand, since the high-temperature reservoir is connected to the heat transfer fluid network containing the pump and the collector channel, at cold weather there is a danger of thermosiphon circulation of the heat transfer fluid from the high-temperature reservoir to the collector channel where the heat is conveyed to the surrounding region.
Here it should be mentioned that in a process of a different class, wherein the energy is obtained exclusively with the aid of the collector channel of a solar collector and the heated heat transfer fluid is supplied by means of a circulation pump via a connecting line to a single reservoir, this circulation and thus the connection to the reservoir is disconnected if the temperature of the heat transfer fluid falls below a predetermined temperature as a result of decreasing sunlight. In this device, a heat valve, whose structure and function correspond to that of the radiator valve of a motor vehicle and which is connected to a short-circuit branch which short-circuits the reservoir, is provided for bridging the cycle running through the reservoir. Accordingly, the circulation pump pumps the heat transfer fluid only through the short-circuit branch. The reservoir is connected again to the cycle only when the heat valve effects the necessary switchover when the predetermined minimum temperature is reached. Such a heat valve switchover has considerable drawbacks. As long as the predetermined temperature has not yet been reached again, the operation of the circulation pump is practically futile. In this case either a timer control for switching on and, when necessary, switching off pump operation is necessary, or additional temperature sensors involving high expenditures have to be used in order to guarantee appropriate pump operation. Moreover, there exists a special drawback in that during times of particularly intensive sunlight the temperature in the reservoir can rise to a value which is considerably higher than the limiting value for disconnecting the bridging short-circuit branch. If the temperature of the entering heat transfer fluid drops to a value slightly above this limiting value, thermal heat fluid of a higher temperature is withdrawn from the reservoir and, instead, heat transfer fluid of a lower temperature is supplied. Thus, on the whole, the reservoir liquid cools down again in a most undesirable manner, without having been utilized for any useful purpose.
There is already known a solar collector device (German Patent DE 3,001,550 Al) in which a low temperature reservoir and a high-temperature reservoir are provided, which are selectively connected to the solar collector in such a manner that the high-temperature reservoir is linked to the solar collector only if there is a growing integral of the measured temperature as well as a positive difference between the temperatures in the solar collector forward-branch and the high-temperature reservoir. If, however, this temperature is not sufficient, a connection to the low-temperature reservoir is carried through. During this operating mode, no switchover to high-temperature reservoir operation is possible, even if the temperature in the forward branch permitted admission to the high-temperature reservoir. If on high-temperature reservoir operation the temperature required for high-temperature reservoir operation drops below the minimum value, a switchover to low-temperature reservoir operation takes place. An automatic switchover to high temperature reservoir operation, if permitted by the irradiation conditions on low temperature reservoir operation, is however not provided, and thus there is no need for heating up the heat transfer fluid having the low-temperature reservoir operation temperature through short-circuit operation before the switchover to high-temperature reservoir operation takes place. In this known design, also first a short-circuit operation during a predetermined delay time takes place after connection, i.e. the collector more or less heats up, for a certain period of time, the heat transfer fluid in the forward/return lines, which has cooled down after the device had been stopped, to a temperature above the temperature of the high-temperature reservoir; the latter is connected to the collector only after expiration of this period.
Further, it has been disclosed in the prior art (German Patent DE 2,554,975 Al) to take the difference between the temperature in the collector chamber and that of the heat transfer fluid, that is, however, the temperature of the fluid entering the collector from the return branch, as a basis for controlling solar collector devices.
Moreover, it is known (European Patent document EP 0,033,756 Al) to use the temperatures of the heat transfer fluid at the inlet and at the outlet of the reservoir, rather than at the inlet and at the outlet of the collector channel, for controlling the reversing valve between a heat reservoir and a collector.
Now it has turned out that a fixed, predetermined temperature differential value that represents an optimum value for operating conditions usually given during certain times of the year may be more or less inappropriate for operating conditions during other times of the year. For intensive irradiation in summer, a small temperature differential value is sufficient; is selecting the same value for operation in winter, however, it often turns out when opening the short circuit branch that the temperature differential value required for transition to high-temperature reservoir operation where the short-circuit branch is closed again is not reached. As a consequence thereof, in this case a switchover to low-temperature reservoir operation is carried through.
In order to appropriately effect a switch over, i.e., an opening of the short-circuit branch, only if in all likelihood a switchover to high-temperature reservoir operation can again take place afterwards, it is advisable to define the predetermined temperature differential value according to specific fixed criteria. In this connection its has proven to be advantageous if the predetermined temperature differential value for reversing the control valve arrangement by opening the short-circuit branch is selected in dependence on the difference in the temperatures of the heat transfer fluid in the high-temperature reservoir and in the low-temperature reservoir.
Experience shows that in winter the difference between the fluid temperature in the high-temperature reservoir and the fluid temperature in the low-temperature reservoir is considerably larger than in summer. Accordingly, the predetermined winter temperature differential value relevant for switchover is higher than the value predetermined for summer.